A fuel system for use in the delivery of fuel to an aircraft engine typically comprises a primary constant displacement fuel pump arranged to supply fuel through a metering valve and a pressure raising and shut-off valve to the burners of an engine. A spill valve is arranged to maintain a substantially constant pressure drop across the metering valve by spilling fuel back to the inlet of the fuel pump. In some arrangements, a second pump is arranged to operate in parallel with the aforementioned primary pump to assist the primary pump during high demand phases of the engine operating cycle.
In order to ensure that a fuel pump is serviced or replaced at the optimum time, it is desirable to be able to monitor the extent of internal wear of the fuel pump without requiring the pump to be removed and dismantled.
Techniques are described in EP 1522731, EP 1138933 and U.S. Pat. No. 5,111,653 whereby the wear status of a fuel pump can be sensed by monitoring of the spill valve operation. However, the arrangements all require special designs of spill valve to be used. As a consequence they are unsuitable for retrofitting into existing fuel systems.